A circuit of this type is known for instance from the European patent application published under No 0201635 which corresponds to commonly assigned U.S. Pat. No. 4,834,818 (J. Pieters-P. Guebels 3-4). It can be used in telecommunication line circuits in order to synthesize suitable signal (AC) impedances presented to various telecommunication lines from an exchange, as well as resistances (DC) to feed the equipment connected to such exchanges via the telecommunication lines, i.e. provide adequate AC and DC terminating impedances under various line conditions. In this known circuit, particular use of a Herter bridge, providing immunity against longitudinal currents, is made in association with a common sense amplifier which is designed to react to a variety of signals having widely differing characteristics such as speech signals, DC signals and relatively high voltage AC ringing currents. This sense amplifier is included in the common loop part and its output feeds separate filter circuits included in both the DC and AC loop parts. In particular, the separate AC loop includes a DC blocking serial capacitance which has a relatively large value for a suitable high pass cut off characteristic eliminating DC but passing the lowest frequency voice signals. Also, in order to cater for the variety of signals that may appear at the input of the sense amplifier, the latter must be carefully designed to avoid saturation by the higher amplitude input signals. Input biassing resistances are used at each of the two inputs terminals of an operational amplifier constituting the sense amplifier. These input biassing resistances are connected to two auxiliary potentials distinct from either ground (0 Volt) or battery potential (-48 Volts), e.g. -33 and -40.5 Volts respectively. As explained in the European application referred to, the first auxiliary potential enables to avoid saturation of the sense amplifier when relatively high voltage ringing signals are applied to the telephone line through the Herter bridge. The second auxiliary potential is used as a special common reference potential for the signals appearing in various parts of the telecommunication line circuit. Since the internal impedances of such auxiliary biassing sources, and particularly the second can never be ideally small, the design must also take care that it will not lead to an unacceptable level of parasitic cross coupled signals.
The circuit according to the published PCT application WO 90/07834 (P. Guebels 8) improves the structure of such an impedance synthesis multiple loop by avoiding previous constraints and more particularly reduces the value of the above filter capacitance, enable a higher AC output from the sense amplifier and avoids a special common reference potential. In this known circuit the filtering means include a transducer fed from the common loop part and with an input stage cascaded with an output stage, an overall negative feedback loop extending from the output of the output stage to the input of the input stage and one of said stages providing to the AC loop part an output signal excluding low frequency components such as DC, the latter appearing only at the output of the other stage feeding the DC loop part.
In this manner, the sense amplifier is now realized as a frequency dependent two-stage amplifier with overall feedback and with the outputs of these two stages supplying directly the low pass, e.g., DC, and high pass (AC) signals respectively.
The input stage of this known circuit further provides the output signal excluding low frequency components such as DC.